BMW, Renault, and Delphi are working together on fuel cell technology.

The concept was announced in April 1999, when BMW and Delphi signed an agreement to develop a vehicle that would use a solid-oxide fuel cell (SOFC) to provide electrical system power. Now Renault has joined the group, accompanied by an announcement that the system will be developed to run with diesel fuel as well as gasoline. This will extend its application into both light- and heavy-duty commercial vehicles and provide the prospect of a solution to forthcoming engine-off legislation. The partners plan to use the fuel cell as an auxiliary power unit (APU), providing the ability to increase the total consumption of electrical energy within the vehicle. As well as feeding the many additional electrical features already introduced, the APU will provide sufficient power for existing mechanically driven subsystems such as the water and air-conditioning pumps to be driven and controlled electrically.

"Generator outputs have had to go up by about 30% and battery capacities by about 200% in the last 30 years," said Burkhard Goeschel, BMW board member responsible for development. "If we reflect that before long our cars will have electric water pumps, electric power steering, and electrically actuated brakes as well as the whole range of modern communication equipment, then we must expect current consumption to at least double again in coming years."

Figures given by Goeschel suggest that power generation of 1 kW (1.3 hp) from a mechanically driven generator currently requires around 1.5 L (0.4 gal) of fuel per 100 km (62 mi). The first production SOFCs will reduce this to around 0.7 L (0.2 gal). Goeschel believes that as electrical requirements soar, this will provide an increasingly significant reduction in fuel consumption and emissions. "This high efficiency translates into better fuel economy which, from Renault's perspective, is especially important for diesel-powered light- and heavy-duty trucks," said Pierre-Alain De Smedt, Executive Vice-President, Renault Group.

Another important use of the APU will be to provide more power than can be reasonably supplied by a battery when the engine is switched off. This power may be used to heat or cool the cabin. "BMW intends to be the first automaker to bring this feature to market for passenger cars," said Goeschel.

In commercial vehicles, an APU could be used to run refrigeration or air-conditioning units while the vehicle is parked overnight or stopped for deliveries. According to Jose Maria Alapont, President of Delphi Europe, the availability of hydrogen in the vehicle will also allow the gas to be mixed with fuel for combustion, creating a more efficient burn and therefore more power and emissions benefits. "The SOFC technology is really quite significant as a way of increasing the overall efficiency of the internal-combustion engine," he said.

The first-generation APU will integrate a reformer and a 108 cell planar SOFC that will provide around 3-kW (4-hp) net output. The system will be integrated within a 42-V electrical architecture. A second-generation system is expected to produce 5 kW (7 hp) within a case that measures approximately 50 x 50 x 25 cm (20 x 20 x 10 in). Hydrogen is generated in the reformer by partial oxidation of gasoline or diesel fuel at 800°C (1470°F). The hydrogen then passes into the SOFC, also running at 800°C (1470°F), where it reacts with oxygen from the atmosphere to generate electricity and water with a potential peak system efficiency of 45%. Waste heat is collected and may be used for supplementary passenger compartment heating, which will be more necessary as engine efficiencies increase. Lithium polymer batteries, also developed and supplied by Delphi, will provide load balancing.

According to Delphi, a major benefit compared with the proton exchange membrane (PEM) fuel cell, which is being widely used for traction applications, is that the SOFC is much less sensitive to impurities. This allows the use of a simple, single-stage reformer and eliminates the need for expensive precious metal electrodes. Delphi also claims to have overcome the SOFC's natural intolerance of sulphur and reports that its zirconium-oxide stacks are currently running well on hydrogen generated from existing commercial-grade fuel. However, Jean Botti, Director of the Customer Solution Centre at Delphi Energy & Chassis Systems, says that Delphi is also developing a PEM fuel cell. "We looked at six different types of fuel cells and concluded that the PEM is currently the right choice for traction applications because of its fast start-up characteristics," he said. "But the SOFC is more attractive as an APU because of its efficiency and because it does not need a cumbersome reforming process to run using current fuels."

Delphi is also working on complementary technologies that could be combined with the SOFC to further increase powertrain efficiency. "We see terrific long-term potential for the SOFC as part of an integrated powertrain strategy," said Botti. "We are already looking at possible integration with our Energen inline starter/alternator technologies to provide fast start, engine damping, and mild propulsion assistance."

Stuart Birch

AEI September 2000
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